1. Field of the Invention
The present invention relates to an automatic image quality-controlling device for use in an FM equalizer in a recording or reproducing circuit for VCRs (video cassette recorders), video-cameras with a built-in VCR (so called camcorder) or the like.
2. Description of the Prior Art
In general, the FM signal used in VCRs (video cassette recorders) or camcorders is composed of a carrier wave and discrete upper and lower frequency bands (lower sideband and upper sideband) from the carrier wave. The frequency range of the carrier wave for the NTSC system is from 3.4 to 4.4 MHz while the frequency range of the carrier wave for the PAL system is from 3.8 to 4.8 MHz. The lower sideband of the signal around 1.5 MHz, governs the resolution of image. When the FM signal of video image is demodulated, the amplitude ratio between the carrier wave and the lower sideband determines the sharpness of the video image. That is, the greater the amplitude of the lower sideband, the clearer the image.
FIG. 1 is a block diagram showing a typical reproducing FM equalizer circuit for conventional VCRs. An FM signal output from a video head 1 is amplified by about 60 dBs in an AGC (automatic gain control) amplifier 2 and the amplified signal is input to a peaking amplifier 3. The frequency characteristics of the video head and video tape decrease with the augmentation of the frequency (see FIG. 2). In order to prevent the occurrence the black-and-white reversal phenomenon and the like, the peaking amplifier 3 subjects the FM signal to a high-frequency compensating peaking treatment as shown by a broken line in FIG. 2. As a result, the output FM signal will have overall frequency characteristics as shown in FIG. 3. This FM signal is input to an FM limiter 4 and then demodulated in an FM demodulator 5. The demodulated signal will be made to undergo a predetermined luminance-signal treatment.
In order to establish the interchangeability between different kinds of apparatuses, it is necessary to secure a margin for preventing the occurrence of the black-and-white reversal phenomenon under the consideration of variations of performances of video tapes used or variations of recording characteristics depending on individual VCRs for tape recording. Therefore, as shown in FIG. 3, the FM signal is emphasized especially in a frequency range of around 5 MHz which governs white display in the luminance signal.
FIG. 4 shows two spectrums of FM signals (as the output signals from the AGC amplifier), one is for a digital signal of the mono-scope pattern etc., and the other is for a typically broadcasted video image. Since the signal of the mono-scope pattern is broadcasted with its lower sideband enhanced, the image is reproduced up to minute part but it will not afford much margins against the black-and-white reversal phenomenon. With regard to the normal image, the signal magnitude is small in the lower sideband, specifically by about 20 dBs at 1.5 MHz as compared to that of the mono-scope pattern. Accordingly, the normal image will present a greater margin for the black-and-white reversal phenomenon.
The characteristic of a reproducing FM equalizer in the VCR is set up such that, for a high-resolution image (a digital image such as of the mono-scope pattern, for instance), the reproduced FM signal is amplified greatly around the frequency range of about 5 MHz that corresponds to white display, in order to inhibit the black-and-white reversal phenomenon from occurring during the demodulation of the reproduced FM signal.
Although this setup inhibits the occurrence of the black-and-white reversal phenomenon in the high-resolution image, the amplitude of the lower sideband becomes relatively low as compared to the amplitude of the carrier wave, thereby the sharpness in the demodulated luminance signal is degraded. On the other hand, as regards the characteristics of video heads as well as video tapes, the noise component increases with the augmentation of the frequency as shown in FIG. 2 and therefore the C/N degrades. As the frequency range (5 MHz) with degraded C/N is made to undergo the peaking treatment, the noise is also enhanced around the peaking range as shown in FIG. 3, thus degrading the S/N of the video output. Since the amplitude of the lower sideband becomes relatively small as compared to the carrier wave, the amplitude of the demodulated luminance signal becomes small in the high-frequency range, as shown in FIG. 5, thus it becomes difficult to produce images with sharpness. Thus, although the FM signal has a relatively great margin against the black-and-white reversal phenomenon, the real situation is such that the sharpness and S/N characteristics in the original signal cannot be utilized efficiently. Further, if a high-resolution reproducing apparatus such as S VHS type apparatuses are used, it is true that the signal is accurately reproduced, but the original source with poor sharpness is reproduced exactly as it is, only to produce unclear images in places of producing a high-resolution image.